The present invention relates to methods and apparatus for condensing a vapor component of a gas mixture and more particularly to the efficient condensation and recovery of solvent vapor from a gas mixture containing such vapor and inert gas.
During the curing of solvent borne resin coatings in a curing oven, the solvent is evaporated into the oven atmosphere. Traditionally, extremely high air flows were forced through such ovens to limit the solvent vapor partial pressure to values below the lower explosive limit of such vapors in air and to remove such vapors from the oven. These prior art curing processes are objectionable in that large air flows remove excessive amounts of heat from the ovens and consequently high levels of fuel consumption are required to continually supply such heat. In addition, the discharge of hydrocarbon based solvents such as acetone, toluene, etc. to the atmosphere degrade air quality. Although it is possible to incinerate solvents in such discharged air streams, additional fuel is frequently required in order to sustain combustion and such solvents cannot be recovered for future use.
Various techniques have been proposed to recover solvent vapors from curing ovens or drying chambers such as the methods and apparatus illustrated in U.S. Pat. No. 4,150,494 which is assigned to the assignee of the present invention. In this patent, liquid nitrogen is utilized as a refrigerant for condensing solvent vapor withdrawn from a curing oven or the like with the resulting, vaporized nitrogen being returned to the oven to inert the same. This enables the recovery of condensed, liquid solvent and avoids the discharge of solvent vapor to atmosphere. The total gas flows through the curing oven are diminished and thus, the quantity of fuel necessary to maintain desired oven temperatures is also reduced. This reference also discloses a single pass heat exchange device for condensing solvent vapors against a countercurrent flow of liquid nitrogen and means for sensing the temperature of the condensed solvent so that the flow of liquid nitrogen to the heat exchange means can be controlled to avoid freezing of the solvent. However, the solvent vapor-inert gas mixture withdrawn from the oven atmosphere generally contains a minor component of moisture, and it is frequently necessary to subject the solvent vapor to relatively low temperatures, below 32.degree. F. to condense acceptable fractions thereof. Moisture will freeze and tend to clog piping or other conduits utilized to recover condensed solvent when such low temperatures are employed. Although the degree of refrigeration supplied to condense solvent vapor may be reduced upon using lower flows of liquid nitrogen, a corresponding reduction in the solvent fraction actually condensed will result and consequently, the non-condensed gas leaving the heat exchange means will contain an unacceptably high partial pressure of solvent vapor. Typically, it is desired to remove by condensation at least 99% or more of the solvent vapor in the withdrawn oven atmosphere. In addition, vaporized nitrogen is returned to the oven through vestibules which tends to increase the difficulty of achieving a mass balance of flows of gas in and out of the oven.
In order to facilitate the condensation of solvent vapor withdrawn from curing ovens in an inert gas mixture, it has been proposed to vaporize a cryogenic liquid to form a cold inert gas (i.e. nitrogen) and utilize the refrigeration in such gas to chill by indirect heat exchange the liquid phase of a solvent. The chilled solvent may then be utilized to condense solvent vapor supplied to a suitable heat exchange device as is illustrated in U.S. Pat. No. 4,237,700 which is also assigned to the assignee of the present invention. However, this system requires relatively large flows of cold, inert gas through an endless conduit and, consequently, is not a particularly effective means for condensing solvent vapor as these systems are bulky and require considerable floor space in a curing plant or facility. A further proposal for utilizing the refrigeration of a cryogenic liquid to condense solvent vapors is described in French published patent application No. 2,349,113. This reference teaches an indirect heat exchange process wherein liquid nitrogen is utilized to condense vapors emitted from a drying chamber. The use of water cooled heat exchange means alone for the purpose of condensing a solvent vapor from a gas mixture is illustrated in U.S. Pat. No. 2,746,168. However, this system is limited in that intense refrigeration is not available from cooling water and frequently intense refrigeration is required in order to condense a high fraction of solvent vapor, typically 99% or more than a gas mixture.
Although the prior art techniques described above provide various approaches to the problem of condensing a vapor component from a gaseous mixture, none of these approaches is fully satisfactory for condensing virtually all of a solvent vapor from an inert gas mixture which may also contain minor components of moisture. Thus, the above noted prior art does not describe efficient processes or apparatus for condensing virtually all of the solvent vapor in inert gas withdrawn from an oven or drying chamber, etc. at a temperature of approximately 250.degree.-600.degree. F. in a safe, reliable and cost efficient method.
One approach toward the recovery of solvent vapor by condensation is described in U.S. Pat. No. 4,444,016, which is assigned to the assignee of the present invention. This approach involves the introduction of a gaseous mixture of solvent vapor and inert gas into a heat exchange vessel which contains a body of the condensed solvent vapor. A stream of cryogenic liquid is introduced directly into this body of condensed solvent vapor to thereby chill the same and enable it to be utilized as reflux liquid. Heat exchange between the reflux liquid solvent and the solvent vapor effects condensation of the latter. The inert gas supplied to the vessel with the solvent vapor is combined with the vaporized cryogenic liquid as an overhead gas and, after being used as a refrigerant in an earlier heat exchange device, is returned to a curing oven to inert the same. However, as the inert gas flow returned to the oven is greater than the flow rate at which inert gas is withdrawn from the oven, it is necessary to control the return flow to maintain a mass balance with respect to the oven. One such control system is illustrated in U.S. Pat. No. 4,475,293 which is also assigned to the assignee of the present invention. Although this control system is effective, equipment in the form of flow transmitters, a ratio controller, etc. is required which in turn increases the overall cost of apparatus for recovering solvent vapor from a particular oven.
Accordingly, there is a clear need for methods and apparatus for recovering solvent vapor from inerted curing ovens or the like by heat exchange with a cryogenic liquid wherein a mass balance between gas flows entering and exiting the curing oven may be readily maintained without requiring costly and complex control equipment.